frequent sql interview questions

difference between ‘where’ and ‘having’ clause

‘where’ clause is used to apply a condition on an column directly.

Usage
select * from table_emp where col_emp_sal<10000;

‘having’ clause is used to apply a condition on a aggregate column.

Usage
select sum(col_emp_sal) as aggr_col_emp_sal from table_emp having aggr_col_emp_sal<10000;

Note: In the above query we cannot use where clause on aggregate alias ‘aggr_col_emp_sal’

difference between ‘distinct’ and ‘group by’ clauses

‘distinct’ keyword is used to remove duplicates
D1.select distinct(col_dept_id) from Table_Employee;  

‘group by’ enables us to use aggregate functions, see in more detail here
G1: select col_sales_date, sum(col_sales) from Table_Sales GROUP BY col_sales_date;
G2: select sum(col_sales) from Table_Sales;

If D1&G2 is considered, both returns the same result but the way they work internally, their execution plans varies across the database servers.

In MS SQL: The execution plan for both the queries is same and hence there wont be any difference
Doing an “EXPLAIN SELECT DISTINCT” shows “Using where; Using temporary ” MySQL will create a temporary table.

vs a “EXPLAIN SELECT a,b, c from T1, T2 where T2.A=T1.A GROUP BY a” just shows “Using where”

difference between UNION and UNION ALL?

UNION Combines Rows From the Two Tables and Excludes all Duplicates Rows in the Result. Execution time wise, this is slower than UNION ALL since it has to eliminate duplicates.
UNION All Returns all Rows in the UNION Operation, including Duplicates. Execution wise it is faster than UNION but it can perform slower at times due to the greater amount of data transferred across the network.